This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.
There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.
Similar to create() in LibJS, wrap() et al. are on a low enough level to
warrant passing a Realm directly instead of relying on the current realm
from the VM, as a wrapper may need to be allocated while no JS is being
executed.
This is similar to how Gecko avoids a reference cycle, where both the
NamedNodeMap and Attribute would otherwise store a strong reference to
their associated Element. Gecko manually clears stored raw references
when an Element is destroyed, whereas we use weak references to do so
automatically.
Attribute's ownerElement getter and setter are moved out of line to
avoid an #include cycle between Element and Attribute.
